iOS Backspace in UITextView substringToIndex , app is crashing .

[int3rc3pt0r]

Hey Guys check this code ( My Custom Keyboard) :

-(IBAction) updateTextBackSpace:(id)sender

{
    if([txtview.text length]>0)
    {
    NSString *deletedLastCharString = [txtview.text substringToIndex:([txtview.text length]-1)];
    [txtview setText:deletedLastCharString];
    }
    else
    {
        NSLog("End");
    }

I am able to erase the letters ( backspace ) but after the first letter is erased and if I press backspace one more time then the application crashes .. AND the main thing is that I can't figure out how to change this code so that , I can erase any text in any give line at the cursor, the backspace starts to erase from end of the line . Please Help